package main;

sub new_foobar_magic {
  my ($self,@otherargs) = @_;
  ...
}

*Foo::Bar::do_magic = \&new_foobar_magic;

# or in one swoop:

*Foo::Bar::do_magic = sub {
  my ($self,@otherargs) = @_;
  ...
}

# if you want to call the old routine:

my $old_foobar_magic = \&Foo::Bar::do_magic;
*Foo::Bar::do_magic = sub {
  my ($self,@otherargs) = @_;
   ...  adjust things or log things ...
  $old_foobar_magic->($self,@args);
};